FIG. 14 is a perspective view showing a wafer stage mounted in a semiconductor exposure apparatus (see Japanese Patent Laid-Open No. 2003-022960).
A wafer stage 300 has a function of holding a wafer (not shown) with a wafer chuck 301a and transporting the wafer to an alignment position or exposure position to align it there. An X-direction coarse-movement beam 303 is arranged on a stage base 310 to be movably guided on the flat surface of the base 310 through a hydrostatic bearing (not shown). The posture of the X-direction coarse-movement beam 303 is held by an X-direction yaw guide 308 in a yaw direction through a hydrostatic bearing (not shown). Accordingly, the X-direction coarse-movement beam 303 is guided to be movable in only an X direction. Similarly, a Y-direction coarse-movement beam 304 is guided by the stage base 310 and a Y-direction yaw guide 309 to be movable in only a Y direction.
Coarse-movement linear motor movable elements 307 using permanent magnets are arranged on the two ends of each of the X- and Y-direction coarse-movement beams 303 and 304. A pair of X-direction coarse-movement linear motor stators 305a and 305b and a pair of Y-direction coarse-movement linear motor stators 306a and 306b are arranged to sandwich the coarse-movement linear motor movable elements 307 in the vertical direction. Each of the coarse-movement linear motor stators 305a, 305b, 306a, and 306b is obtained by winding a coil around an iron core formed by stacking comb-shaped thin silicon steel plates. A magnetic attracting force acts between each iron core and the corresponding coarse-movement linear motor movable element 307. The coarse-movement linear motor movable element 307 is sandwiched by the coarse-movement linear motor stators at equal gaps in the vertical direction to cancel the attracting force.
When a current is appropriately supplied to the coils of the coarse-movement linear motor stators 305a, 305b, 306a, and 306b, thrusts can be generated between the coarse-movement linear motor stators and the coarse-movement linear motor movable elements 307. The coarse-movement linear motor stators are formed on the same structure as that of the stage base 310, and the thrusts act in the respective moving directions of the X- and Y-direction coarse-movement beams 303 and 304. Corner cubes (not shown) are provided to the X- and Y-direction coarse-movement beams 303 and 304 to reflect laser beams from laser interferometers (not shown). The positions in the respective moving directions of the X- and Y-direction coarse-movement beams 303 and 304 are measured by the laser interferometers. A control system (not shown) controls positioning of the X- and Y-direction coarse-movement beams 303 and 304 with the measurement values of the respective laser interferometers and the coarse-movement linear motors.
An X-Y slider 302 is arranged to surround the X- and Y-direction coarse-movement beams 303 and 304. The weight of the X-Y slider 302 is received by the stage base 310 with a hydrostatic bearing (not shown) provided to the bottom plate of the X-Y slider 302. Hence, the X-Y slider 302 is guided on the stage base 310 to be movable within an X-Y plane. Noncontact guides are formed between the X-Y slider 302 and the X- and Y-direction coarse-movement beams 303 and 304. The noncontact guides can comprise either static guides or electromagnetic guides.
Furthermore, a fine-movement stage 301 is mounted on the X-Y slider 302. The fine-movement stage 301 is mounted on the X-Y slider 302 by insulating vibration from below by a pneumatic spring or a self-weight compensating system which employs magnetic repulsion. Single-phase linear motor stators comprising coils are provided to the X-Y slider 302, and single-phase linear motor movable elements comprising permanent magnets are provided to the fine-movement stage 301. The single-phase linear motors can apply thrusts in the X, Y, and Z directions and in ωx, ωy, and ωz directions which are the rotational directions about the X-, Y-, and Z-axes to the fine-movement stage 301. For example, three single-phase linear motors may be arranged at positions that do not line up straightly in the Z direction to apply thrusts in the Z, ωx, and ωy directions. Two single-phase linear motors may be arranged in the X direction and two single-phase linear motors may be arranged in the Y direction to apply thrusts in the X, Y, and ωz directions.
A laser reflection mirror (not shown) is provided to the fine-movement stage 301 to reflect the laser beams from the laser interferometers (not shown), so as to measure the positional displacement in 6 degrees of freedom (X, Y, Z, ωx, ωy, and ωz) of the fine-movement stage 301. Position control systems (not shown) are provided to the X- and Y-direction coarse-movement beams 303 and 304 and fine-movement stage 301, respectively, and give appropriate commands to the X and Y linear motors and single-phase linear motors to accurately align the wafer stage at an arbitrary position within a movable range.
The imaging characteristics of the projection optical systems and the measurement accuracy of the leaser interferometer are largely adversely affected by the apparatus and a change in ambient temperature. When the ambient temperature changes, the laser beam of the laser interferometer fluctuates to degrade the measurement accuracy. When the temperature changes, the member to which a mirror as the measurement target of the laser interferometer deforms. Then, the positions relative to each other of the mirror as the positional reference and the substrate change to degrade the measurement accuracy simultaneously. Particularly, today an alignment accuracy on the order of nanometer (nm) is required. For example, a 100-mm low-expansion member (with a thermal expansion coefficient of 1×10−6) deforms by 100 nm with a temperature change of 1° C. Even when a change in air temperature in the laser optical path of the laser interferometer is 1° C., the measurement value of the position may change by 100 nm depending on the conditions. Hence, it is significant to maintain the constituent members of the projection exposure apparatus and the ambient temperature constant.
The fine-movement stage described above is driven highly accurately at high speed and with high acceleration. Thus, the load acting on the actuator such as the linear motor that drives the fine-movement stage increases, and the necessary current increases accordingly to increase the heat generated by the coil. As the wafer size increases, the stage size also increases. Thus, the load acting on the actuator increases and the necessary current increases to increase the heat generated by the coil. When the ambient temperature is changed by the generated heat, the laser beam of the laser interferometer fluctuates to degrade the measurement accuracy. The member (fine-movement stage) to which the mirror as the measurement target of the laser interferometer deforms. Then, the positions relative to each other of the mirror as the positional reference and the substrate change to degrade the measurement accuracy simultaneously.
To avoid heat generation, the movable element is desirably made lightweight. In view of this, a structure is proposed as described in Japanese Patent Laid-Open No. 2003-163257 (see FIGS. 2 to 6), in which a top panel serving as a stage constituent member is bored to form a hollow structure so as to decrease the weight, and ribs are arranged in the top panel to maintain high rigidity. With the method of forming the top panel to have the hollow structure in this manner, to obtain required rigidity, the side plates which surround the top panel have large thicknesses. Consequently, the heavy top plate is not worth the improvement of rigidity.
In the trend for a larger wafer size, the alignment apparatus shown in FIG. 14 which has a 2×2 matrix structure formed of X- and Y-direction beams becomes very bulky, and the following problems arise accordingly.    It is difficult to obtain desired stage acceleration.    Heat generated by the linear motors during acceleration increases to make it difficult to ensure high exposure accuracy.    As the mechanical natural frequency of the stage decreases, it becomes difficult to ensure a servo band.
Due to the above problems, with the conventional arrangement, to realize a desired specification is becoming difficult. In order to provide a stage apparatus that copes with an increase in wafer size, a guideless flat stage having 6 degrees of freedom is proposed, which can perform alignment with a long stroke in X-Y planar directions, in addition to the tilting and Z directions (for example, see Japanese Patent Laid-Open No. 2004-254489).